The difficulty in building FE's is something I never understood. Maybe it's because I've been around them since the 60's that they are just another chunk of iron to me. As with all engine builds there are little "tricks" that you perform but they are just what you do. There are certainly things that you look out for and certain things to take into account in the physical difficulty of these engines. I guess the main things to watch for are rear main seal installation, chamfering and aligning some of the
oil passages and getting the intake to sit squarely on squarely mounted heads. Setting a 4bbl intake over the fenders on a pick up can be quite challenging but there's even little tricks to that I use. Other than a few design interchange issues, it's all righty tighty, lefty loosey. There's no great mysteries.